store模块阅读9:PutMessageLock,PutMes

说明

这一节讲解存放消息时涉及到的锁

PutMessageLock :锁的接口
PutMessageReentrantLock:可重入锁的实现
PutMessageSpinLock: 自旋锁的实现

源码很简单直接贴

源码

接口PutMessageLock

public interface PutMessageLock {
    void lock();

    void unlock();
}

重入实现PutMessageReentrantLock

public class PutMessageReentrantLock implements PutMessageLock {
    private ReentrantLock putMessageNormalLock = new ReentrantLock(); // NonfairSync

    @Override
    public void lock() {
        putMessageNormalLock.lock();
    }

    @Override
    public void unlock() {
        putMessageNormalLock.unlock();
    }
}

自旋实现PutMessageSpinLock

public class PutMessageSpinLock implements PutMessageLock {
    //true: Can lock, false : in lock.
    private AtomicBoolean putMessageSpinLock = new AtomicBoolean(true);

    @Override
    public void lock() {
        boolean flag;
        do {
            flag = this.putMessageSpinLock.compareAndSet(true, false);
        }
        while (!flag);
    }

    @Override
    public void unlock() {
        this.putMessageSpinLock.compareAndSet(false, true);
    }
}

思考

为什么存放消息需要锁，而MappedFile中shutdown却不需要锁

MappedFile中对改变内容的只有追加新内容,不会修改内容或者删除内容,互斥的是存放消息即可

而对于shutdown, ReferenceResource#shutdown执行了之后会导致ReferenceResource#hold返回false
使得不会有后续的MappedFile中的selectMappedBuffer,commit,flush的行为发生
即在代码控制了shutdown之前的操作都有效，之后的操作均无效

重入锁和自旋锁的区别

网上资料很多，简单来说，自旋锁不能先锁两次再释放两次，而重入锁可以